167 



Podopsis. Shell inequivalve, subregular, adhering by the inferior beak, 

 not auriculated, the lower valve largest, most convex, and the beaks most 

 produced; hinge without teeth. Ligament internal. 



2 fossil species. 



OSTRACEA.— Five Genera. 



This family is separated into two divisions. 



DIVISION I. Ligament internal, or demi-internal. Shell irregular, 

 foliated, sometimes very thin. 



DIVISION II. Ligament demi-internal. Shell foliated, and often 

 very thick. 



Gryphea. Shell free, inequivalve, the lower valve large, concave, ter- 

 minating by a projecting, involute beak, the upper valve small, flat, and oper- 

 cular; hinge without teeth; an oblong, arched, cardinal pit; only one muscular 

 impression in each valve. 



G. angulata, 11 fossil species. 



Ostrea. Shell adhering, inequivalve, irregular; beaks distant, becoming 

 very unequal by age, and the upper valve gradually during the life of the 

 animal; hinge without teeth. Ligament demi-internal, inserted in the cardi- 

 nal pit of the valves; the pit of the lower valve increasing by age, sometimes 



